In fuel feed pumps in automobiles, there has been known a so-called in-tank-type system having a motor and a pump integrated in a fuel tank, in which a commutator of the motor directly contacts the fuel because the fuel in the tank is fed through a motor housing to outside devices. In automobiles using gasoline including alcohol, the problem arises that copper in the sliding member of the commutator, which contacts with the brush, is eroded by the alcohol in the gasoline. To avoid this, Japanese Patent Application, First Publication No. Hei 8-308183 discloses a carbon commutator which contains carbon in the member for sliding on the brush.
In the carbon commutator described in the publication, a plurality of segments (commutator pieces), which are produced by compacting and sintering a carbon powder, are arranged to be insulated from each other in a circumferential direction on an insulating boss member made of a synthetic resin. Copper riser pieces which are conductive terminal members are integrally sintered together with the segments. This publication discloses that, in order to ensure electrical connection between the conductive terminal members and the segments, conductive metal powder may be compacted and sintered around the conductive terminal members, or a mixture of the metal powder and the carbon powder is stratified and sintered so that the ratio of the carbon powder may be increased from the conductive terminal members toward the sliding member for the brush. In order for the coefficient of thermal expansion to approximate that of the riser pieces, the same copper material as the riser pieces or nickel-plated copper powder is selected for the metal powder.
However, the above copper powder and the nickel-plated copper powder are not integrated with the riser piece during sintering in a range of 700.degree. C. to 900.degree. C., which is the sintering temperature range for the carbon powder. The riser pieces merely come in contact with the copper powder in the sintered segments, which may therefore become unstable. It is known that the green compact of the metal powder contracts during the sintering, so even when the same copper material as the riser pieces is selected as the metal powder to approximate the coefficient of thermal expansion, the problem arises that gaps may be occur between the sintered compact of the copper powder and the riser pieces.
When the carbon powder and the copper powder are stratified and sintered, binder added to the carbon powder is thermally decomposed and carbonized so that in the carbon powder portion the contraction ratio is greater than the expansion ratio. The contraction ratio of the copper powder is less than the contraction ratio of the carbon powder portion, while the thermal expansion in the copper powder during the sintering is greater than that in the carbon powder. Therefore, slippage may occur at the boundary between the carbon powder and the copper powder, so the sintered segments are apt to separate at the boundaries. It is impossible in the conventional carbon commutator to achieve a long-term stable electrical and mechanical connection especially in fuel which contains alcohol as mentioned above.
It is therefore an object of the present invention to provide a carbon commutator which contains carbon in the sliding members and which achieve long-term stable electrical and mechanical connection between the segments and the conductive terminal members.